Method, apparatus, and system for producing buckypaper or similar sheet or layer of elongated nanostructures with a degree of nanostructure alignment

ABSTRACT

A method, apparatus, and system for fabricating buckypaper or similar sheets of nanostructures having relatively high aspect ratios. A dispersion of nanostructures such as nanotubes is subjected to fluid dynamics/forces which promote alignment of their axes of elongation while in suspension in the flow. An agglomeration of better aligned nanostructures is isolated from the carrier fluid into a useable form. In the case of nanotubes, one form is buckypaper. One example of alignment forces is Taylor-Couette flow shear forces. One example of isolation is filtering the flowing dispersion to collect better aligned nanostructures across the filter into a sheet or film. The degree of alignment can produce anisotropic material properties that can be beneficially used in application of the sheet or film.

I. CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Divisional Application of U.S. Ser. No. 14/334,755, filed Jul.18, 2014, which is herein incorporated by reference in its entirety.

II. BACKGROUND OF THE INVENTION A. Field of the Invention

The present invention relates to fabrication of buckypaper or ananalogous sheet or layer with some degree of alignment of nanostructureshaving some elongation in at least one direction and, in particular, toutilization of fluid flow dynamics to influence filtered collection ofthe nanostructures from a suspension of the nanostructures.

B. Related Art

Nanostructures are materials with at least one dimension on the order ofnanometers in scale. Much work is being done to develop applications forthem, either individually or in agglomeration. Nanostructures withelongation in one direction can be fibers (nanofibers) or otherstructures having an axis of elongation or an aspect ratio well above 1.One example is tubes (nanotubes). Another example is nanocellulosefibrils. Another example is nanoribbons.

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindricalnanostructure. They are members of the fullerene structural family andresemble graphitic sheets rolled into tube shapes. Rolling angle andradius influence their properties. They have been found valuable for,inter alia, nanotechnology, electronics, optics and other fields ofmaterials science and technology.

Both single-walled (SWCNT) and multi-walled (MWCNT) varieties of CNTshave been synthesized and both exhibit outstanding mechanical, thermal,and electrical properties. These extraordinary properties have prompteda great deal of research into the efficient synthesis of CNTs, and todayhigh quality MWCNTs are commercially available at huge quantities andlow cost (˜$120/kg). As a result, high-volume engineering applicationsof MWCNTs are becoming a reality after decades of promise at thelaboratory scale. One of the most promising immediate applications ofMWCNTs is as filler in composites, specifically polymer matrixcomposites. However, composites fabricated by mixing multi-walled carbonnanotubes (MWCNTs) into a resin are limited to low loading levelsbecause the large increases in viscosity that occur at higher loadingsencumber processing. This, in turn, limits the effect that MWCNTs canhave on the composite properties, and new methods must be developed ifthe true potential of CNT composites is to be realized. One way toachieve high loadings of CNTs in a composite is through the use ofbuckypaper (BP), which is a free-standing mat of tightly packed CNTsformed by the controlled filtration of CNT solution. FIGS. 1A and 1Bshow the typical method used to produce BP and optical images of the BPitself. BP can be handled in a manner similar to glass and carbon fibermats, and traditional composite processing techniques such ascompression molding and vacuum-assisted resin transfer molding can beused to infiltrate resin into the pores of the BP mat and bind severalplies together into composites. See references[3, 4] (bracketed numbersrefer to the listing of References later in this description). See alsoL. Hussein et al., Phys. Chem. Chem. Phys. 13, 5831 (2011), incorporatedby reference herein. Loadings up to 60 wt % MWCNTs have been achieved[5] and outstanding mechanical, [6,7] thermal, [8] electrical, [7] andelectromagnetic shielding properties [9] have been realized in BP/epoxycomposites. Most BP is composed of CNTs that are randomly aligned.However, as with any fiber-reinforced composite, optimal properties arerealized when the fiber alignment is unidirectional within each ply andthe composite layup is judiciously tailored to match the expected stressstate of its application.

However, as well-recognized by those in this technical field, theextremely small size of nanostructures and their properties presentsubstantial challenges regarding their handling. What might work withmacro-sized discrete items may not work with nano-sized structures.

Three approaches currently exist for the production of aligned BP mats:alignment through mechanical stretching of cross-linked CNT mats,“domino pushing” of aligned CNT forests, and magnetic alignment.Mechanical stretching involves uniaxially straining randomly alignedMWCNT BP and then impregnating it with resin. Bismaleimide (BMI)/BPcomposites made with this process possessed outstanding mechanical andelectrical properties, [5] and, when the MWCNTs in the BP werefunctionalized with epoxide groups, the resulting composites exhibitedunprecedentedly high strength (3081 MPa) and modulus (350 GPa),surpassing even high-performance carbon fiber composites. [10] Seeillustration at FIGS. 2A and 2B and [34] (see D. Wang et al.Nanotechnology 19, 609 (2008), incorporated by reference herein).However, the MWCNTs used in this study were cross-linked togetherthrough a specialized synthesis process necessary to prevent the BP fromtearing at high strains, which excludes the method from widespreadindustrial use in the near future. Additional discussion of mechanicalstretching can be found at Cheng Q, Bao J, Park J, Liang Z, Zhang C,Wang B. High Mechanical Performance Composite Conductor: Multi-WalledCarbon Nanotube Sheet/Bismaleimide Nanocomposites. Advanced FunctionalMaterials. 2009; 19(20):3219-25 [5] (use of mechanical stretching ordrawing to align CNTs), which is incorporated by reference herein.

Highly aligned BP can also be produced through “domino pushing” of MWCNTforests. See illustrative at FIG. 3. In this method, vertically alignedMWCNT forests are grown on a Si substrate and are subsequently pushedover by physically rolling over the forest with a cylinder. BP producedin this way has higher electrical and thermal conductivity in thedirection of alignment. [20] However, this method is also not amenableto large-scale use, as MWCNT forests with very high degrees of verticalalignment must be grown, a process that is currently only possible in afew laboratories. Ding W, Pengcheng S, Changhong L, Wei W, Shoushan F.Highly oriented carbon nanotube papers made of aligned carbon nanotubes.Nanotechnology. 2008; 19(7):075609. [20] (use of mechanical rolling or“domino pushing” to align CNTs) gives further discussion of thisapproach, and is incorporated by reference herein.

Magnetic alignment is an alternative method developed by Smalley [21]and refined by Liang and coworkers. [22] This method involves filteringCNTs in the presence of an applied magnetic field. Because CNTs haveanisotropic magnetic susceptibilities, they tend to align with thedirection of applied magnetic field lines in order to minimize energy.See illustration at FIG. 4. If a sufficiently strong magnetic field isapplied to MWCNTs that are very well dispersed in solution, the MWCNTswill become oriented, and subsequent filtering will lead to theformation of aligned BP. Individual nanotubes comprising a MWCNT can bemetallic or semiconducting depending on their structure withparamagnetic or diamagnetic responses to applied magnetic fields,respectively, both of which tend to align the MWCNT in the samedirection and with nearly the same force. [23-25] However, huge magneticfields on the order of 10-30 T are required to produce observabledegrees of alignment. [21] The cryogenically-cooled electromagnetsneeded to achieve those massive magnetic fields render this method unfitfor the production of aligned BP on any appreciable scale. Additionaldiscussion of the magnetic alignment approach can be found at publishedpatent application US 2002/0185770 to McKague (use of magnetic fields toalign CNTs), which is incorporated by reference herein.

McKaque U.S. 2002/0185770 describes some of the issues in this technicalfield, including the challenges faced trying to achieve CNT alignment.McKaque discusses the potential benefits from such things as anefficient way to produce mass quantities of BP; with an effective degreeof CNT alignment to produce anisotropic and other beneficial properties,and the ability to produce composites containing improved loadings ofCNTs relative to simple melt or resin mixing.

Thus, the state of the art recognizes there is a need for alignedbuckypaper fabrication techniques. But as discussed above, suggestedsolutions leave room for improvement in terms of flexibility,efficiency, complexity, economy, and applicability to a wide range oftypes of nanostructures.

III. SUMMARY OF THE INVENTION

It is therefore a principle object, aspect, advantage, or feature of theinvention to provide methods, apparatus, and systems for improving overthe state of the art.

Further objects, aspects, advantages, or features of the invention areto provide a method, apparatus, or system for producing BP or analogousend products with elongated nanostructures fixed in some directionalalignment which:

-   -   a. is relatively economical and non-complex;    -   b. works for most, if not all, types of NTs as well as at least        some other elongated nanostructures;    -   c. can be scaled up or down to produce different sized BP sheets        or similar agglomerations; and/or    -   d. has flexibility and adjustability regarding a number of        parameters, including degree of directional alignment in the        resulting agglomeration.

In one aspect of the invention, a method of creating a macro-scale mator sheet made of elongated nanoscale structures from the Fullerenestructural family includes creating a dispersion of nanostructure andfluid with the nanostructures at a predetermined dilution, using fluidflow dynamics to influence some degree of alignment of thenanostructures in the fluid, and agglomerating or aggregating theinfluenced nanostructures into the mat or sheet. At least in the case ofCNTs, compared to the starting random orientations, a degree ofalignment can produce beneficial material properties. Examples caninclude but are not limited to anisotropic electrical and mechanicalproperties.

One specific example of practicing the above method is filtering thedispersion to substantially block and collect the nanostructures acrossa macro-scale area but allowing passage of the remainder of thedispersion after or while subjecting the dispersion to shear forces. Thecombination of actions on the dispersion promotes shear thinning of thedilution and at least a degree of alignment of the nanostructures alongtheir axes of elongation as the nanostructures are formed into themacro-scale mat or sheet (BP for CNTs).

One example of a method and apparatus of imparting shear thinning iswith Taylor-Couette flow. Filtering can be through a porous section ofthe inner tube of a Taylor-Couette set-up. Collection of thenanostructures by progressive build-up on top of the filter produces thesheet or mat. The method can be scaled within reason. Variables can beadjusted to influence the amount of alignment in the sheet or mat.

In another aspect of the invention, an apparatus for making buckypaperor the like of nanostructures includes at least one surface or boundaryalong which a dispersion of elongated nanostructures and fluid can flow.A flow generator is controllable to generate a shearing rate in thedispersion effective to promote preferential alignment of thenanostructures in the flow. An agglomeration or aggregation componentisolates the nanostructures into a layer that can be processed into afree-standing sheet.

In one specific example, the apparatus can have an elongated axisincludes a fluid chamber; a fluid outlet from the chamber; a fluidpermeable surface between the fluid chamber and the fluid outlet; meansor components to generate fluid flow dynamics that tend to influencenanostructure alignment (one example being fluidic shearing forces at ornear the permeable surface) when the chamber contains a dispersion ofthe nanostructures and a carrier fluid.

In one specific apparatus, the shearing forces can be produced in aTaylor-Couette type two concentric cylinder assembly. At least one ofthe cylinders is operatively connected to an actuator or motor to rotateit relative to the other at a speed designed to set up shear stress inthe direction of the desired alignment of CNTs in the dispersion at andacross a filtering section of the inner cylinder.

In another aspect of the invention, a sheet of elongated nanostructuresis made by the process of providing a dispersion of fluid and thenanostructures, influencing nanostructure alignment with fluid flow, andforming an agglomeration or aggregation of aligned nanostructures thatcan be processed into a free-standing mat or sheet.

In one specific example, the agglomeration is formed by placing a filterover a fluid outlet; directing the dispersion to the filter; andcontrolling flow of the dispersion to create shear forces substantiallyin one direction at or near the filter; so that the nanostructures areinfluenced to align in the direction and deposit in a layer on thefilter. The deposited layer is removed from the filter to isolate asheet of elongated nanostructures (BP for NTs) with a degree ofalignment. The sheet can be utilized in a wide variety of applicationssimilar to other BP but has both fabrication benefits over existingstate-of-the-art techniques as well as the benefits of anisotropy for atleast one material property

IV. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram of a conventional set-up to producerandomly-aligned buckypaper.

FIG. 1B is an enlarged picture of several sheets of BP produced by thesystem of FIG. 1A.

FIG. 2A is a photograph at greatly enlarged scale of a firstconventional way of producing aligned BP (stretching/pulling).

FIG. 2B are photographs of the stretching process and how the stretchedfibers can be assembled into plies or multi-layers.

FIG. 3A is a diagram illustrating a second conventional way of producingaligned BP (domino rolling).

FIG. 3B is a diagram of how a sheet of aligned CNTs is removed from theset up to produce BP.

FIG. 4 is a highly diagrammatic view of a third conventional way ofproducing aligned BP (magnetic alignment).

FIG. 5A is a flow chart of a method of creating aligned BP according toone exemplary embodiment and aspects of the present invention.

FIG. 5B is a highly diagrammatic illustration of how the method of FIG.5A is implemented to create aligned BP or analogous product according toaspects of the present invention.

FIG. 6A is a front elevation view of a modified Taylor-Couette devicefor use in a specific exemplary embodiment according to the presentinvention.

FIG. 6B is a diagrammatic view of a full system set up for producing BPusing the modified Taylor-Couette device of FIG. 6A.

FIG. 6C is a simplified 2-D diagram of the modified device of FIG. 6A.

FIG. 6D is a perspective view diagram illustrating fluid flow in thedevice of FIG. 6A.

FIG. 6E is a cross-section view of FIG. 6D diagramming shear stressforces that can influence nanoparticle alignment in the fluid flow undercertain conditions.

FIG. 6F is a diagram characterizing flow regimes for Taylor-Couettecylinders rotating at different velocities and resulting fluid dynamics,including discovered beneficial flow regimes applied to the device ofFIG. 6A.

FIG. 6G is a perspective, partially sectioned diagram of flow patternsgenerated in a Taylor-Couette device such as FIG. 6A, and a 2-Dprojection of local flow dynamics inside the device for certainconditions.

FIG. 7 is an enlarged plan view of a sheet of BP produced by theapparatus and system of FIGS. 6A and 6B.

FIG. 8A is a graph illustrating shear thinning produced by the apparatusof FIG. 6A.

FIG. 8B is similar to FIG. 8A but showing shear rates can differ as afunction of temperature or concentration of CNTs.

FIG. 9A is a set of greatly enlarged plan view photographs showing atmicroscopic scale different degrees of alignment for different shearrates for BP produced by the setup of FIG. 6B.

FIGS. 9B and 9C are plan view photographs similar to FIG. 9A showingunaligned (FIG. 9B) and a degree of alignment (FIG. 9C) of BP.

FIG. 9D are plan view microphotographs similar to FIG. 9A but showingback or opposite views.

FIG. 10A is a graph illustrating demonstrated anisotropic electricalproperties of aligned BP under certain conditions, as compared tonon-aligned BP.

FIG. 10B is an illustration of how measurements such as FIG. 10A aremade along and transverse to the direction of alignment in the BP.

FIG. 11 is a graph illustrating demonstrated anisotropic electricproperties of aligned BP under different shear rates produced in adevice like FIG. 6A.

FIG. 12 is a graph demonstrating anisotropic mechanical properties foraligned BP in comparison to non-aligned.

FIGS. 13A-C are graphs illustrating different anisotropic mechanicalproperties for aligned BP versus non-aligned BP.

V. DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION A.Overview

For a better understanding of the invention, one or more exemplaryembodiments of forms the invention can take will now be described indetail. It is to be understood that the invention can take many formsand embodiments and those described below are neither inclusive norexclusive.

The examples will be discussed in the context of CNTs, either SWCTs orMWCTs. However, the invention can be applied to other nanostructuresthat have elongation along one axis, including those with relativelyhigh aspect ratios. Examples are nanofibers, nanotubes, nanoribbons, andnanocellulose. These categories include, for example, carbon nanofibers,boron nitride nanotubes, titanium dioxide nanotubes, zinc oxidenanowires, semiconducting nanowires (e.g. Si, InP, GaN), metal disulfidenanotubes, metal nanowhiskers, metallic nanowires (e.g. Ni, Pt, Au),insulating nanowires (e.g. SiO₂, TiO₂), and molecular nanowires (e.g.DNA).

Furthermore, the examples are illustrated in the context of a batch-modeproduction of BP, in the sense that individual single BP sheets areproduced one-at-a-time. However, a continuous or at leastsemi-continuous process is possible. One example of filteringnanostructures from a dispersion is set forth in U.S. Pat. No.7,459,121, incorporated by reference herein, which could be applied torecovery of aligned nanostructures from the present embodiment.

B. Generalized Method and Apparatus

FIG. 5A describes anisotropic fabrication of a layer or sheet ofelongated nanostructures by use of fluid flow dynamics to promote somedegree of alignment along their axes of elongation. Instead ofstretching, rolling, or magnetically inducing alignment of thenanostructures during production of BP, the method suspends thenanostructures in a water-based carrier fluid. Controlled fluid flowinfluences entrained elongated nanostructures towards alignment in thedirection of flow is the technique of promoting alignment.Nanostructures with some degree of alignment, relative to their originalrandomly-oriented state, are then available for isolation, harvesting,collection in as much of that alignment state as possible for theformation of BP in the case of NTs, or in an analogous sheet, film,layer, or the like for other nanostructures with an aspect ratiosignificantly greater than 1.

The suspension originally includes a dilution of the nanostructures inthe fluid at a pre-determined concentration. An example is NC7000multiwall carbon nanotubes (MWCNTs) commercially available from Nanocylof Sambreville, BELGIUM, but a variety of elongated nanostructures canbe utilized. As a general rule, the type of elongated nanostructuresrelevant to the methodology tend to be those with aspect ratiossignificantly greater than 1. While not so limited, one range mightinclude aspect ratios on the order of 2:1 and above.

The nanostructures begin randomly oriented and well dispersed in thefluid. One way to do so is by sonicating the CNTs before mixing into thefluid, and composing the fluid of distilled water with a surfactant. Anexample of a surfactant is Triton X-100 commercially available from DowChemical Company, Midland, Mich., USA, but a variety of others areavailable and possible. The surfactant chosen can depend on the materialbeing suspended and the fluid. The surfactant can beneficially have onecomponent having favorable interaction with the material and onecomponent with favorable interaction with the fluid. The “component” isusually a portion of the molecule (functional group, chain, or branch).Amphiphilic molecules are often good choices, as they have twocomponents in the same molecule: one that is hydrophilic and one that ishyrdrophobic.

Using fluid flow dynamics, at least a substantial number of thesuspended microscopic elongated nanostructures are influenced towardsalignment in the same or similar direction at least to some degree morethan the random orientation in the starting fluid. One way to promotealignment is with fluid flow dynamics. The processing can be an appliedlaminar shear flow. One fluid dynamic that could be used is shearforces. Shear forces tend to arise in fluid flowing along a constrainedboundary. See FIG. 5B. Fluid along the interface with the boundary tendsto flow slower than fluid away from it. This creates velocity variancesin the direction of flow. At sufficient differences, shear forces aregenerated. The small but elongated nanostructures can be influenced bythe shear forces to align in the direction of flow. The designer canselect the flow technique and control parameters to influence alignment.Another example can be other methods of creating laminar flow thatinfluences at least a substantial number of nanostructures into somedegree of alignment. Empirical testing may be required to find thebetter control parameters for a needed or desired amount of alignment.

Once an effective degree of alignment of the nanostructures in the fluidis achieved for a given application, the nanostructures can be collectedfrom the flowing fluid. One example is a free-standing sheet or layer(in the case of NTs formation of BP). Other elongated nanostructuresshould react similarly. Again, the designer can select the method andcontrol parameters for this step. And other methods are possible. Theparadigm shift from conventional methods allows such technical benefitsas:

-   -   a. Non-complexity. The handling of nanostructures is not easy        because of their extremely small size. Suspension in fluid        allows them to be transported en masse while entrained in a flow        of the fluid for processing using relatively non-complex and        economical components.    -   b. Scale of processing. The scale of processing is adjustable        within practical limits. Processing can be done one sheet at a        time or plural processing paths in parallel. This improves over        some conventional methods that presently have limitations on the        size (length and width) of the sheet that can be produced.    -   c. Type of nanostructure. The process can be applied to most if        not all types of nanostructure having a direction or axis of        elongation in the sense that if fluid flow dynamics can        influence some degree of alignment of orientation on the        nanostructure, although aspect ratios of at least 2:1 may react        better.    -   d. Adjustability. Because fluid flow dynamics can affect degree        of alignment of the nanostructures, process controls can be        implemented to produce BP or the like (sheet, mat, film, etc.)        of different degrees of nanostructure alignment. Different        degrees of alignment can produce different material properties.        Examples include electrical, mechanical, and thermal properties,        to name a few. And there can be meaningful anisotropy in at        least one or more of those properties when compared parallel to        direction of alignment versus off-parallel, and at least        parallel versus perpendicular to direction of alignment. This        allows the designer of the BP to have some control over degree        alignment for different needs or desires.    -   e. Flexibility. At least the foregoing benefits afford the        designer flexibility in design of sheets of the nanostructures        (e.g. BP for NTs).    -   f. Economy. At least the foregoing benefits can be implemented        in cost-effective components and processing methods.

As mentioned, one way to influence direction of nanostructures insuspension is by shear forces. Fluid constrained by at least oneboundary generated shear when flowing. A volume of nanostructure/fluiddispersion is flowed or otherwise moved along such a surface. Boundaryconditions generate forces that are exerted on the suspendednanostructures moving in the fluid. Examples of such forces are: shearcreated when fluid is forced through a confined opening (e.g. die) orpipe or when a fluid is confined between parallel plates and the platesare moved relative to each other.

In one example, once moving along the boundary, under certain conditionsshear forces act to set up fluid flow dynamics which, if appropriatelydesigned and controlled, result in flow patterns that influencedirectional alignment of the nanostructures in the fluid.

Agglomeration, harvesting, or collection of these more alignednanostructures (as compared with their starting orientations) across anarea fabricates a sheet or mat of the nanostructures having some degreeof alignment. In most examples, the agglomeration relies at least inpart on inherent self-adhering nanostructure properties. One example ofagglomeration, harvesting, or collection is shown in FIG. 1A. Some typeof filter media of a given area (length and width) would be imposedalong the fluid flow. The flow would be, at least in part, directed,influenced, or assisted through the filter media to catch and separateat least a substantial quantity of elongated nanostructures from thesuspension to build up a free-standing sheet or film of nanostructures,and with a degree of alignment of the nanostructures instead of therandom the orientations of FIG. 1A. Other techniques for agglomerating,aggregating, harvesting, or collecting could be centrifugal orgravitational separation.

Thus, in one example, the nanostructure/fluid dispersion is subjected tofluid flow shear and shear stress. Those dynamics are controlled to actupon the nanostructures to tend to align at least a substantial numberof them. The dispersion or suspension is filtered to collect the alignednanostructures across a filtered area. The collected nanostructures forma layer or sheet which can be isolated. One form of isolation of thesheet is to remove it without tearing, stretching, or otherwisematerially affecting its desired properties. It is to be understood,however, that other methods of influencing alignment and harvestingnanostructures out of a flowing dispersion are possible.

Therefore, using fluid flow forces can influence alignment ofnanostructures in suspension. This is a relatively inexpensive andnon-complex method as compared to any of the three main present methodsdiscussed in the Background of the Invention.

This is amenable to scaling up or down according to need or desire byvarying the area of the filter.

It is amenable to an automated production process for commercialquantities of aligned sheet material.

Also, control of the process allows at least some control over theamount of alignment in the material.

FIG. 5B is a highly diagrammatic view of an apparatus according toaspects of the invention. The pre-mixed dilution of randomly-orientednanostructures in a carrier fluid is subjected to fluid flow dynamics bywhat will be called a flow generator effective to align a substantialnumber of nanostructures. They are then agglomerated by what will becalled an agglomeration or aggregation component over an area to createa sheet or film of nanostructures with some degree of alignment comparedto random orientation in the starting dispersion. One way is to collectthe nanostructures from the flowing fluid over a filter area.

In one example, flowing fluid relative the boundary wall generatedforces in the fluid flow act which influence alignment upon themicroscopic nanostructures. One example can be setting up shear forcesin the direction of flow as diagrammatically indicated.

A filter would be designed to stop (be impermeable to) at least asubstantial amount of the nanostructures while allowing (be permeableto) the fluid to pass. Therefore, a sheet of nanostructures (e.g. BP forNTs) is collected on the filter.

But alternatives are possible to take advantage of using nanostructuresin fluid suspension as a starting material for fabricating sheets likeBP with some degree of nanostructure alignment.

Importantly, entraining the nanostructures in fluid and directing flowof the fluid is a scalable process that does not require the machineryand limitations of the stretching, rolling, or magnetic-field generationof the state-of-the-art methods discussed above.

A. Specific Example 1

A specific exemplary embodiment of a device and system to practice thegeneralized method described above is illustrated with reference toFIGS. 6-13 and any subparts. It is to be understood this is but onespecific form the invention can take.

Anisotropic Buckypaper Through Shear Induced Mechanical Alignment ofCarbon Nanotubes in Water

1 ABSTRACT

A simple method for aligning nanotubes in buckypaper (BP) with amodified Taylor-Couette system is reported. Using shear forces producedby a rotating cylinder to orient multi-walled carbon nanotubes (MWCNTs)in a surfactant-assisted aqueous dispersion, the suspended nanotubes aresimultaneously aligned and filtered. The resulting BP is composed ofnanotubes with preferential orientation in the direction of flow andpossesses anisotropic electrical and mechanical properties, which areboth enhanced parallel to the direction of orientation. The techniquepresented here requires no specialized equipment and can be implementedwith any type of carbon nanotube (CNT) synthesized by any method.Furthermore, the size of the BP sheets can be easily increased byadjusting the length and diameter of the cylinders in the setup,offering the possibility for low-cost production of large quantities oforiented BP.

2 INTRODUCTION

Carbon nanotubes (CNTs) have been studied extensively over the last twodecades because of their outstanding electrical, mechanical, and thermalproperties, which makes them ideal candidates for use as reinforcementin multifunctional composites among other applications. [1, 2] However,composites fabricated by mixing CNTs into a resin are limited to lowloading levels because the large increases in viscosity that occur athigher loadings encumber processing. This, in turn, limits the effectthat CNTs can have on the composite properties, and new methods must bedeveloped if the true potential of CNT composites is to be realized. Oneway to achieve high loadings of CNTs in a resin is through the use ofbuckypaper (BP), which is a free-standing mat of tightly packed CNTsformed by the controlled filtration of a CNT dispersion. BP can behandled in a manner similar to glass and carbon fiber mats, andtraditional composite processing techniques such as compression moldingand vacuum-assisted resin transfer molding can be used to infiltrateresin into the pores of the BP mat and bind several plies together intocomposites. [3, 4] Using this approach, composites containing up to 60wt % MWCNTs have been achieved[5] and outstanding mechanical,[6, 7]thermal,[8] electrical,[7] and electromagnetic shielding properties[9]have been realized in BP-reinforced polymers. Most BP is composed ofCNTs that are randomly aligned. However, as with any fiber-reinforcedcomposite, optimal properties are realized when the fiber alignment isunidirectional within each ply and the composite layup is judiciouslytailored to match the expected stress state of its application.

Several methods to align CNTs within BP films have been reportedrecently, which can be broadly classified as alignment through (i)mechanical stretching of cross-linked CNT mats, (ii) pushing or pullingvertically-aligned carbon nanotubes (VACNTs), and (iii) the applicationof large magnetic fields. Mechanical stretching involves uniaxiallystraining randomly aligned multi-walled carbon nanotube (MWCNT) BP andthen impregnating the stretched nanotube film with resin. Bismaleimide(BMI)/BP composites made with this process possessed outstandingmechanical and electrical properties,[5] and, when the MWCNTs in the BPwere functionalized with epoxide groups, the resulting compositesexhibited unprecedentedly high strength (3081 MPa) and modulus (350GPa), surpassing even high-performance carbon fiber composites.[10]However, the MWCNTs used in this study were cross-linked togetherthrough a specialized synthesis process necessary to prevent the BP fromtearing at high strains, which excludes the method from widespreadindustrial use in the near future.

Highly aligned BP can also be produced from VACNT arrays, which consistof forests of densely-packed and highly aligned nanotubes. By pulling ona VACNT forest, van der Waals attraction among neighboring nanotubescauses the CNTs to assemble into continuous yarns or BP mats. [11-19] Inaddition to being spun by pulling action, VACNT forests can also be“pushed” down like dominos to form BP. This method has been implementedfor vertically aligned MWCNTs using a cylinder to physically roll overand flatten the nanotube forest, and the BP produced in this mannerexhibited higher electrical and thermal conductivity in the direction ofalignment. [20] However, this method is also not amenable to large-scaleuse, as MWCNT forests with very high degrees of vertical alignment mustbe grown, a process that is currently only possible in a fewlaboratories.

Magnetic alignment is another nanotube orientation technique developedby Smalley [21] and refined by Liang and coworkers. [22] This methodinvolves filtering CNTs in the presence of an applied magnetic field.Because CNTs have anisotropic magnetic susceptibilities, they tend toalign with the direction of applied magnetic field lines in order tominimize energy. If a sufficiently strong magnetic field is applied toMWCNTs that are very well dispersed in solution, the MWCNTs will becomeoriented, and subsequent filtering will lead to the formation of alignedBP. Individual nanotubes within a MWCNT can be metallic orsemiconducting depending on their structure with paramagnetic ordiamagnetic responses to applied magnetic fields, respectively, both ofwhich tend to align the MWCNT in the same direction and with nearly thesame force. [23-25] However, huge magnetic fields on the order of 10-30T are required to produce observable degrees of alignment. [21] Thecryogenically-cooled electromagnets needed to achieve those massivemagnetic fields render this method unfit for the production of alignedBP on any appreciable scale.

An alternative approach for aligning nanotubes in BP is outlined here.When subjected to shear forces in a fluid, CNTs align along thedirection of flow. Using a modified Taylor-Couette system, an aqueousMWCNT dispersion is simultaneously sheared and filtered to produce BPwith preferential nanotube orientation in the direction of flow. Thealigned BP has anisotropic electrical and mechanical properties, whichare both enhanced parallel to the direction of orientation. Thetechnique presented here is simple and versatile in that it can beadapted for use with nanotubes synthesized by any method. In addition,the size of the BP can easily be increased using cylinders with largerdimensions. As a result, this approach offers an attractive route forproducing large quantities of oriented BP at relatively low cost.

5.3 Experimental Details

5.3.1 Materials

NC7000 MWCNTs with an average diameter of 10 nm and purity of 90% weresupplied by Nanocyl, S. A. (Belgium). A surfactant, Triton X-100 waspurchased from Fisher Scientific (Waltham, Mass., USA). Nanotubedispersions were prepared by sonicating mixture of 1.5 g MWCNTs, 15 mLsurfactant, and 750 mL DI water with a horn (Fisher, sonic dismembratormodel 100) for 2 hours. The resulting dispersion was allowed to settlefor 24 hours, and the well-dispersed supernatant was used to prepare BPfilms with a setup shown schematically at reference number 10 in FIGS.6A and 6B. See FIG. 6A—Schematic of modified Taylor-Couette system usedto simultaneously shear and filter MWCNT dispersions and FIG. 6B, abench-top set up including the concentric tube apparatus 10.

5.3.2 Methods

The modified Taylor-Couette apparatus 10 was constructed from an acrylicouter cylinder 12 with a length of 30 cm and an inner diameter of 31.15mm, and a high-density polyethylene Porex (Fairburn, Ga., USA) innercylinder 14 having a length of 31 cm, an outer diameter of 26.00 mm andan average porosity of 60 μm. The inner cylinder 14 was sealed withadhesive tape 15 along its length, except over an 8 cm long section. Anelectric stirring motor 16 (Caframo, Ontario, Canada) with rpm controlof ±1 rpm was used to rotate the outer cylinder at speeds up to 2000 rpm(e.g. by turning an axle 18 fixed to the bottom of outer tube 12).Compression fitted PTFE bushings 20T (top) and 20B (bottom) secured tothe inner cylinder 14 maintained the inner tube 14 parallel to the outercylinder 12 while allowing the two to be separated easily. A small filltube 22 was inserted into a slit milled in the top bushing 20T toprovide fresh dispersion during filtration, and a vacuum in the innercylinder 14 was created by a belt-driven pump 24 (Welch, Niles, Ill.,USA) via vacuum line 25. In all experiments, the top of the innercylinder 14 was clamped to prevent rotation.

To fabricate BP using the setup 10, a 9 cm×8 cm strip of nitrocellulosefilter paper 30 (Osmonics, Inc.) with an average pore size of 45 μm wasaffixed to the exposed porous section of the inner rod 14 by pulling thepaper 30 tautly around the cylinder 14 and bonding the overlapping endsof the paper 30 with a small amount of adhesive. After the adhesive hadcured, the inner cylinder 14 was inserted into the outer cylinder 12 andthe gap 28 was filled with dispersion 32.

The outer cylinder 12 was rotated at a constant rate to shear the fluid32, and vacuum was subsequently applied to the fixed inner cylinder 14to force the dispersion through the filter paper 30. A BP sheet (e.g. BPsheet 34 of FIG. 7) formed on the filter 30, and the separated water wascollected in a series of traps 36 (FIG. 6B). Fresh dispersion 32 wascontinually added from a container 38 of reserve dispersion via the filltube 22 during filtration to maintain a constant fluid level in gap 28.

After filtration, the inner cylinder 14 was removed from the setup andthe filter paper 30 was cut along the overlapped edge to produce arectangular sheet, which was dried in a vacuum oven at 100° C. for 12hr. The dried BP 34 was then separated from the filter paper 30 bygently folding and peeling the nanotube mat 34 free from the filter 30.The resulting freestanding film of MWCNTs 34 was soaked in isopropanolovernight to remove any residual surfactant before drying once more in avacuum oven at 80° C. for 4 hr. FIG. 7 shows a representative sample ofBP 34 after processing. See, e.g., FIG. 7—Optical image of dried BPsheet formed under a shear rate of 1000 s⁻¹.

5.3.3 Characterization

The viscosity of the MWCNT dispersions was measured as a function ofshear rate using an AR2000ex rheometer equipped with a Peltiertemperature control stage and a 40 mm diameter cone (α=1°, 0′, 11″).Measurements were performed by placing 0.2 mL of dispersion 32 on thePeltier stage and equilibrating at 25° C. before performing a constanttemperature, steady-state flow test at shear rates ranging from 1 to1200 s⁻¹.

The degree of nanotube alignment in BP samples was monitored withscanning electron microscopy (SEM, FEI Quanta 200) operating at 8 kVaccelerating voltage. Electrical conductivity measurements wereperformed using a linear four point probe (Jandel model RM2). For eachtest, a 1 cm×3 cm strip was cut from the BP either perpendicular orparallel to the direction of alignment, and conductivity measurementswere made on the top surface of the paper with the four probes orientedparallel to the long axis of the strip. The thickness of each sample wasaveraged from 10 measurements taken along the length of the BP 34 usinga digital micrometer (Mitutoyo). The anisotropic mechanical propertiesof BP samples were evaluated by tensile testing 0.5 cm×3 cm×100 μmstrips cut either parallel or perpendicular to the direction ofalignment. For each test, the BP strip was mounted in a flat-facedfixture and elongated with an Instron universal testing machinefollowing a procedure similar to ASTM D882.

This alternative approach to the conventional ways of producing alignedBP is also illustrated at FIGS. 6C-G. It involves the use of shearforces to align MWCNTs in solution as they are being filtered. Likeother fibers, CNTs align along the direction of flow 50 in a fluidsubjected to shear 52. When a MWCNT solution is sufficiently dilute andwell dispersed, shear thinning behavior is observed as fibers align andreduce drag (See FIG. 8B—illustrating shear thinning behavior of aqueousCNT dispersions as a function of temperature and concentration. [26]).

A classic setup to generate shear forces in fluids is the Taylor-Couettesetup, which is shown schematically in FIG. 6C (See also FIGS. 6D and6E—Schematics of Taylor-Couette cylinder. See also White, F M. FluidMechanics: WCB/McGraw Hill; 2003, including Chapter 4, ISBN:0077422414).

B. Operation

By rotating the outer 12 and/or inner cylinder 14, shear forces developin the fluid trapped between the two cylinders, and the magnitude of theshear is determined by both their radii and relative speeds. The shearrate produced by rotating only the outer cylinder 12 is given by: [28]

$\overset{.}{\gamma} = {{\frac{{dv}_{\theta}}{dr} \approx \frac{V_{o} - V_{i}}{R_{o} - R_{i}}} = {\frac{R_{o}\omega}{R_{o} - R_{i}} = \frac{\omega}{1 - {R_{i}/R_{o}}}}}$where {dot over (γ)}, R_(o), R_(i), and ω are the average shear rate,radius of outer cylinder, radius of inner cylinder, and angular velocityof the outer cylinder, respectively. Rotation of the outer cylinder 12is desirable for aligning fibers in solution as it avoids turbulenttransitions that can occur from instabilities associated with rotationof the inner cylinder 14.

FIG. 6F (diagram of flow regimes for Taylor-Couette cylinders rotatingat different velocities. [29]) shows the turbulent flow regimes thatdevelop when the outer and inner cylinder are rotated at differentrelative Reynolds numbers, which are proportional to velocity. The arrow40 represents the case of only inner cylinder rotation, and clearlyturbulence occurs at much lower velocities than the case of only outercylinder rotations (shown at arrow 42), in which laminar (Couette) flowis observed up to very high Reynolds numbers on the order of10,000-15,000. See Coles D. Transition in Circular Cuoette Flow, Journalof Fluid Mechanics. 1965; 21(03): 385-425, which is incorporate byreference herein.

We have made aligned BP sheets by building a Couette-Taylor setup withan inner cylinder composed of Porex© polymer and the outer cylinderpolycarbonate as shown in FIG. 6A and schematically in FIG. 6C. Theouter cylinder 12 is rotated with an overhead mixing motor 16 capable ofspeeds up to 2000 rpm with digital rate control within ±1 rpm. Filterpaper 30 is placed over the porous section of the Porex inner cylinder14, and the top is connected to a vacuum line 25. A well-dispersedsolution 32 of MWCNTs prepared by sonicating MWCNTs and Triton X-100(surfactant) in distilled water is poured into the gap 28 between thetwo cylinders 12 and 14. The outer cylinder 12 is rotated to align thenanotubes through shear and vacuum is subsequently applied to filter theMWCNTs and form BP 34 on the inner cylinder 14. Various rotationalspeeds have been used to generate shear rates on the orders of 100-1000s⁻¹.

The degree of anisotropy in the resulting BPs has been characterizedwith FE-SEM and four point probe conductivity measurements both paralleland perpendicular to the direction of alignment. In the future,composites will also be made from the BP by compression molding andvacuum infiltration of epoxy, and tensile testing and DMA (dynamicmechanical analysis or sometimes dynamic mechanical spectroscopy such asis known in the art) will be performed to determine mechanicalproperties as a function of orientation. See, e.g., Menard, Kevin P.(1999). “4”. Dynamic Mechanical Analysis: A Practical Introduction. CRCPress. ISBN 0-8493-8688-8, incorporated by reference herein. Somepreliminary results are summarized below.

C. Anisotropic Benefits

FIG. 10A (Representative I-V curve obtained from four point probemeasurements of randomly oriented BP) is a representativecurrent-voltage (I-V) plot obtained from a four point probe measurementon non-oriented BP. FIG. 10B diagrammatically illustrates how theconductivity measurements were made on the BP sheet. The BP behaves asan Ohmic conductor as can be seen by the linear relationship betweencurrent and voltage, and, as a result, the electrical conductivity ofthe BP can be obtained from the slope of the I-V plot. Table 1summarizes the conductivity of BP samples produced at differentrotational speeds as measured both parallel (σ_(//)) and perpendicular(σ_(⊥)) to the direction of shear alignment. Randomly oriented BPproduced by filtration without rotation had nearly isotropic electricalconductivity, as determined by the similar values of σ_(//) and σ_(⊥).An intermediate level of shear generated by rotating the outer Couettecylinder 12 at 500 rpm produced the greatest electrical anisotropy(σ_(//)/σ_(⊥)˜2), while samples produced under 1000 rpm rotation werenearly isotropic. The higher shear rate produced visible turbulence inthe MWCNT solution, which likely led to random deposition of MWCNTs onthe filter paper. Optimization of the shear rate is part of an ongoinginvestigation.

TABLE 1 Summary of anisotropic electrical conductivity of BP filteredunder rotation at 0, 500, and 1000 rpm. Sample σ_(//)(S/cm) σ_(⊥) (S/cm)σ_(//)/σ_(⊥) Random 21.4 ± 0.8 19.2 ± 0.4 1.11 ± 0.1  500 rpm 33.6 ± 3.416.9 ± 2.0 2.01 ± 0.4 1000 rpm 24.6 ± 2.2 21.3 ± 0.7 1.16 ± 0.2

The BP discussed in this section was made using inner and outercylinders with different diameters than that done with the cylinderdimensions described in experimental section above). The results inTable 1 and FIG. 9C are given based on rotational speeds instead ofshear rates. These results are consistent with the other results. SEMimages of the random and aligned BP are given in FIG. 9B (SEM image ofrandomly aligned BP formed by filtration without rotation) and FIG. 9C(SEM image of BP formed under rotation at 500 rpm. The arrow indicatesthe direction of alignment during filtration), respectively. Some degreeof alignment seems apparent in FIG. 9C in the direction of shear, whileFIG. 9B appears to contain MWCNTs with random orientation. In summary,we have developed a new and simple method of producing anisotropic BPthrough orientation and filtration of an aqueous MWCNT solution.Preliminary results have shown that BP formed at intermediate shearrates have anisotropic electrical properties, which could be of greatvalue for use in composite and electronic applications.

Taylor-Couette flow and shearing action are well known and described inthe literature. See, e.g., [28] (Darby) and [29] (Anderack),incorporated by reference herein.

FIG. 6D diagrammatically illustrates how flow 50 in gap 28 betweencylinders 12 and 14 is parallel the circumference of inner tube 14 (andgenerally perpendicular to the longitudinal axis of inner cylinder 14).Flow is a function of relative radius R₁ of cylinder 14 and R2 ofcylinder 12, and relative angular velocities Ω₁ and Ω₂ respectively.FIG. 6E illustrates the shear forces 52 in direction of flow (atvelocity ν_(θ)). It can be seen that shear forces 52 are along thecircumferential flow direction.

FIG. 6F illustrates that Taylor-Couette flow must be controlled to avoidsubstantial turbulence. It also shows that rotation of the outercylinder is preferred over rotation of the inner cylinder, as that willavoid turbulent transitions that will reduce the degree of nanostructurealignment relevant to nanostructure alignment influence by the flow.Arrow 42 in the figure shows the flow regime (Couette a.k.a. laminar)that occurs when the outer cylinder is rotated. Arrow 40 shows the flowregimes (many different turbulent types) that occur as the rotationalspeed of the inner cylinder is increased.

FIG. 6G illustrates diagrammatically and with an isolated, expanded flowdiagram from a vertical section of gap 28, and turbulent flow patternsthat develop in the fluid if the inner cylinder is rotated too quicklyand the viscosity of the fluid is low. It shows the fluid flow regime“Taylor vortices” that are described in FIG. 6F.

4 RESULTS AND DISCUSSION

4.1 Rheological Behavior of the MWCNT Dispersion

FIG. 8A depicts the rheological behavior of the MWCNT dispersion used inthis study. In a similar manner to previous reports on aqueous nanotubedispersions, [26, 27] the viscosity was observed to decreasesignificantly with increasing shear rate. This shear thinning behavioris due to the fact that MWCNTs align under shear, which lowers theirresistance to flow. The viscosity of the dispersion used in this studyplateaus at ˜800 s⁻¹, indicating that the nanotubes reach their maximumdegree of alignment at shear rates above this value. See FIG.8A—Rheological behavior of the aqueous MWCNT dispersion used in thisstudy. See also FIG. 8B, which includes the data of FIG. 8A but showshow thinning behavior varies as a function of temperature andconcentration of the nanostructures in the fluid (pH=6.0).

4.2 Fabrication of Aligned BP

The Taylor-Couette setup is a classic method for studying fluid behaviorunder shear. By rotating the outer and/or inner cylinder, shear forcesdevelop in the fluid trapped between the two cylinders, the magnitude ofwhich is determined by both their radii and relative speeds. [28]Rotation of the outer cylinder is desirable for aligning fibers insolution as it avoids turbulent transitions that can occur frominstabilities associated with rotation of the inner cylinder. [29] Themodified Taylor-Couette setup used in this study was designed to produceshear rates from 0 s⁻¹ to 1200 s⁻¹. By shearing the dispersion whilesimultaneously applying a vacuum to the inner cylinder, the suspendednanotubes were circumferentially aligned and then forced onto filterpaper. Progressive build-up of MWCNT layers led to the formation of BPcomprised of nanotubes with a preferential orientation parallel to thecircumference of the cylinders. In this description, the direction offlow is referred to as “//”, and the direction perpendicular to flow(the axial cylinder direction) is referred to as “⊥”. The morphology ofsamples produced at various shear rates is depicted in FIG. 9A, in whichall of the micrographs were collected from the front side of the MP(side adjacent to cylinder gap). With no cylinder rotation (0 s⁻¹), thenanotubes are randomly oriented. As the shear rate is increased to 640s⁻¹, the nanotubes become partially oriented in the // direction. At aneven higher shear rate of 825 s⁻¹, the MWCNTs are highly aligned in thedirection of flow. Shear rates above 825 s⁻¹ also produced BP withMWCNTs oriented in the // direction, although a higher degree alignmentis not discernible. Notably, the degree of alignment appears to varysomewhat through the thickness of the BP film. FIG. 9D showsrepresentative micrographs of BP produced at different shear rates asviewed from the backside of the films (side adjacent to filter paper).The degree of nanotube alignment seems diminished compared to the frontside, possibly due to interactions among the nanotubes and filter paperfibers. See FIG. 9A—Scanning electron micrographs of BP formed at shearrates of 0 s⁻¹, 640 s⁻¹, 825 s⁻¹, and 1000 s⁻¹.

4.3 Electrical Conductivity of BP

While SEM indicates alignment of MWCNTs in BP produced at elevated shearrates, it is a qualitative measure. To better quantify the degree ofanisotropy, the electrical conductivity of BP samples was measured indifferent directions. FIG. 10A shows representative current-voltagecurves obtained from four point probe measurements of ˜100 μm thick BPproduced at shear rates of 0 s⁻¹ and 1000 s⁻¹. BP formed at all shearrates displays Ohmic behavior. However, random BP fabricated in theabsence of shear exhibits very little directional dependence, while BPformed at a shear rate of 1000 s⁻¹ has markedly lower slope (V/I) in the// direction and higher slope when measured ⊥ to alignment as a resultof higher and lower conductivity, respectively. FIG. 10B illustrates themanner in which the measurements were taken.

FIG. 11 summarizes the electrical behavior of BP produced in this studyas a function of shear rate and measurement direction. At low shearrates, the electrical anisotropy, defined as the ratio of conductivitymeasured // and ⊥ to alignment, is ˜1. With increasing shear rate, theconductivity steadily increases // to alignment while decreasing in the⊥ direction, and the anisotropy ratio reaches a plateau of ˜2 around 825s⁻¹. This behavior coincides with a plateau in shear thinning observedby rheology, and suggests that viscosity measurements are a convenientmethod for determining the minimum shear rate needed to maximizenanotube alignment. See FIGS. 10A-B—Representative I-V curves for BPsamples produced in the absence of shear (dashed lines) and at shearrates of +1000 s⁻¹ (solid lines) measured both parallel (solid square orcircular data points) and perpendicular (open square or circular datapoints) to the direction of flow. See also FIG. 11—Summary of electricalconductivity measurements performed on BP in directions parallel (σ//)and perpendicular (σ⊥) to the direction of flow at various shear rates(an anisotropy ratio of the two (σ///σ⊥) is also depicted. That ratio isan indication of the degree of anisotropy of the material. As can beseen, anisotropy is maximized above 825 s⁻¹ shear rates. Also σ///σ⊥plateaus at a value of approximately 2.

Conductivity in BP is generally dictated by nanotube-nanotube junctions,which limit the mean free paths of electrons and lower the conductivity.[30, 31] The BP produced in this study does not contain perfectlyaligned MWCNTs, and as a result, nanotube-nanotube junctions play a rolein the conductivity in all directions. However, electrons traveling inthe // direction of aligned BP samples will encounter far fewerjunctions than electrons traveling in the transverse direction, and, asa result will experience less resistance. Anisotropic electricalconductivity has been observed in aligned BP samples produced by othermethods, and Table 2 (below) compares the results of this study with aselect number of those previously reported in the literature. Here wereport a maximum anisotropy of ˜2, which is lower than that achievedusing magnetic alignment and pulling of VACNTs, but similar to the valuefound by “domino pushing” MWCNT forests.

The lower levels of anisotropy found in this study may be due to thepresence of a higher number of misaligned nanotubes than by magneticalignment. Because the relaxation time of water is very short, theaqueous dispersions used in this study may have allowed some MWCNTs torelax and coil upon removal of shear forces, especially on the upper fewlayers, which are less constrained by neighboring nanotubes. The use ofhigher viscosity fluids may limit such relaxation and improve nanotubealignment and packing density. Greater levels of alignment might also beachieved by tuning the interaction among the nanotubes, surfactant, andfilter paper, and greater electrical properties could be realized byextending the approach to other varieties of CNTs such as high aspectratio SWCNTs.

TABLE 2 Literature reports of electrical anisotropy in aligned BP atroom temperature σ// Method (S/cm) σ⊥ (S/cm) σ///σ⊥ Reference MagneticAlignment SWCNT 1100 138 8.0 [32] SWCNT 1210 200 6.1 [33]Pushing/pulling VACNT MWCNT 209 110 1.9 [34] MWCNT 403 56 7.2 [31]Mechanical Stretching Nanocomp MWCNT* 600 — —  [5] oriented BP *Studydid not report σ⊥ but did find that σ// was 40% higher than randomlyoriented BP4.4 Mechanical Properties of BP

The anisotropic mechanical properties of BP produced in the absence ofshear and at a shear rate of 1000 s⁻¹ were also investigated to test theefficacy of BP for composite applications. FIG. 12 shows representativestress-strain curves for BP tested at both shear rates in differentdirections. The results of testing many samples are summarized in FIGS.13A-C. Randomly oriented BP has a modulus of ˜0.4 GPa, ultimate tensilestrength near 4 MPa, and a strain at break of 1% with no directionaldependence within experimental error. In contrast, BP produced underhigh shear shows strong anisotropy, with moduli and tensile strengths2.8 and 2.2 times higher in the direction of alignment, respectively.However, even in the direction of alignment, the mechanical propertiesare modest, and further improvements are likely needed before the BP canbe considered for use as reinforcement in composites. See FIG.12—Representative stress-strain curves for BP produced in the absence ofshear (0 s⁻¹) and at {dot over (γ)}=1000 s⁻¹ in directions both paralleland perpendicular to flow. See also FIGS. 13A-C—Summary of mechanicalproperties for BP prepared in the absence of shear (0 s⁻¹) and at 1000s⁻¹ in directions both parallel (//) and perpendicular (⊥) to flow.

5 CONCLUSIONS

A simple method for aligning nanotubes in BP with a modifiedTaylor-Couette system is here reported. Simultaneous shear-alignment andfiltration of an aqueous MWCNT dispersion yielded BP with preferentialnanotube orientation in the direction of flow. The BP exhibitedanisotropic electrical and mechanical properties, which were bothenhanced parallel to the direction of orientation and maximized at highshear rates. While the highest degree of anisotropy was found to belower than some previously reported methods, such as magnetic alignment,the technique presented here is simple and versatile in that it can beadapted for use with any type of CNT synthesized by any method. Inaddition, large BP sheets can be easily fabricated by increasing thelength and diameter of the cylinders in the setup, making this approachan attractive route for the producing large quantities of oriented BP atrelatively low cost.

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D. Options and Alternatives

It is to be appreciated that the invention can take many forms andembodiments and that the exemplary embodiments neither limit nor definethe scope of the invention, which is defined by the appended claims.Variations obvious to those skilled in the art are included within theinvention.

For example, the generalized and specific examples given above provideillustrations of some of the ways the invention can be implemented. Thedesigner can vary one or more aspects according to need or desire.

For further example, as mentioned, the specific apparatus to influencenanostructure alignment is not necessarily limited to Taylor-Couettedevices. And even with Taylor-Couette devices, variations are possible.One variation would be to vacuum-assist filter the dispersion ofnanostructures at the outer cylinder (fixed) instead of the innercylinder (rotating). A porous or fluid permeable section of the outercylinder could be covered by filter medium to collect thenanostructures. Still further, it may be possible to filter at both theinner and outer cylinders and one or the other, or both, could berotating. Another variation would be filtering without vacuum-assist.And, as mentioned, the dimensions and operational variables of theTaylor-Couette apparatus such as apparatus 10 can be varied according toneed or desire. Longer cylinders, perhaps with a larger gap 28 between,could be configured to produce larger area size BP or the like sheets.The device 10 could also be scaled down from the dimensions indicated toproduce a smaller area sheet. The designer can tune or select from thevarious design parameters to affect such things as density or howtightly packed the aggregated nanostructures are in the free-standingmat or sheet, the degree of preferential alignment of nanostructures,and the size and even shape of the mat or sheet. The designer can selectattributes about the system from at least the following general rulesbut variations from them are possible:

-   -   a. Gap width: The gap between inner and outer cylinders is a        function of the cylinder radii and the minimum and maximum        ratios of the inner cylinder to the outer cylinder can be 0.70        to 0.99. The gap width can vary, but the larger the gap, the        lower the shear stress and the faster the cylinders must be        spun. Therefore, rotational speed generally governs the gap size        chosen. There may be an upper limit to the gap size that must be        avoided to prevent turbulence. Indications are that keeping the        radii ratio <0.7 should avoid that problem.    -   b. Radii of cylinders: The minimum and maximum radii of the        inner cylinder can be between approximately 1 mm and 2 m.    -   c. Speed of rotation: The rotational speed of the outer cylinder        typically could range from 100-2500 rpm. The range discussed        above may or may not work with different radii cylinders and gap        sizes. It would be dependent upon_the shear rate desired.    -   d. Resident time. Typical time of operation to extract a        buckypaper sheet would be on the order of 60 to 300 seconds.        However, greater or less resident time of the dispersion in the        gap would result in thicker or thinner BP. The system typically        is operated for a sufficient time to build up a sufficient        density of nanostructures to form a tightly-packed free-standing        layer when removed from the filter paper. The basic rule to        decide how long the system should be operated for that purpose        depends on, for example, i) the ease with which the BP can be        separated from the filter membrane (generally the minimum is 15        μm to allow for easy separation) and (ii) the desired thickness        for a given application (longer filtration produces thicker BP.        The basic range of density of nanostructures would be on the        order of 0.5-1 g/cm³ per. The basic range of thicknesses of the        final layer would typically be between 15 and 500 μm. The        minimum thickness can be dictated by the mechanical properties        of the BP: a minimum of about 15 um is generally indicated for        easy separation of the BP from the filter. The maximum thickness        is generally a function of filtration time and vacuum power.        Longer time and stronger vacuum are indicated to lead to thicker        deposition, although the rate of thickness increase will likely        decrease as the BP gets thicker due to the fact that it too acts        as a filtering medium (increasing the effective filter        thickness).    -   e. Filter characteristics. The range of average pore size is        dependent upon the size of the nanostructures and the desired        filtration speed. And the offset between the average pore size        of the porous section of the inner cylinder and the average pore        size of the filter paper is not critical—the inner pore size can        be larger but typically should not be significantly smaller than        the filter pore size in order to maximize BP formation speed.        The pore size of the filter is important. It is indicated that        it should have pore size large enough to allow fast fluid flow        but small enough to prevent excessive loss of nanostructures        through the filter. The filter in one embodiment is unwoven.    -   f. Amount of vacuum-assist. A range of between 650 and 125 torr        is believed sufficient for effective vacuum-assist. However, it        may be possible to collect the nanostructures without        vacuum-assist.    -   g. Materials. Basic general rules regarding the material        properties of the inner and outer cylinders is that at least the        walls containing the dispersion should have an absolute        roughness of on the order of 0.0015 to 0.1 mm, a coefficient of        friction of on the order of 0.1 to 0.6.    -   h. Tightly-packed self-standing sheet or mat. By tightly-packed        it is meant that the density per square cm of nanostructures        ranges from 1 to 1.3 mg/cm^(2.) By self-standing it is meant        that the BP is unsupported and can be handled in ways analogous        to paper including rolling, folding, etc.    -   i. Shear rates. The foregoing description gives examples of        shear rates deemed effective for an effective amount of        preferential alignment of nanostructures. It is to be        understood, however, there could be situations where shear rates        below those to achieve maximum alignment are desirable. But        typically, shear rates of between 500 s⁻¹ and 1200 s⁻¹ should        achieve beneficial preferential alignment in the flowing fluid        for most nanostructures of the type discussed herein.    -   j. Degree of alignment. This can be difficult to quantify so one        way is to use anisotropic property measurements to indirectly        define extent of alignment. For instance, electrical and        mechanical anisotropy measurements (e.g. such as described        earlier) give a sense of the degree to which the nanotubes are        aligned. One goal for an embodiment of the invention could be a        degree of anisotropy in the approximate range of 1 to 2 between        directions parallel and perpendicular to an axis of alignment.

What is claimed is:
 1. An apparatus for making a mat or sheet ofnanostructures each having an elongated axis comprising: a. at least onesurface with a boundary along a flow path which a dispersion ofelongated nanostructures and fluid can flow; b. a flow generatorcontrollable to generate a fluid flow effective to promote preferentialalignment of the nanostructures in the flow; and c. an agglomeration oraggregation component operatively associated with the boundary whichisolates the nanostructures into a layer; d. so that the layer can beprocessed into a free-standing mat or sheet.
 2. The apparatus of claim 1wherein the agglomeration or aggregation component comprises a fluidpermeable surface between the fluid chamber and the fluid outlet,wherein the boundary comprises a section of the fluid chamber and theflow generator generates fluidic shearing forces at or near thepermeable surface when the fluid chamber contains the dispersion.
 3. Theapparatus of claim 1 wherein the flow generator comprises a modifiedTaylor-Couette system comprising: a. concentric outer and inner tubesforming a fluid chamber therebetween; b. at least one of the inner tubeand the outer tube operatively connected to an actuator to rotate theouter tube relative to the inner tube at different rotational speeds ordirections to generate Taylor-Couette flow imparting shear stress onfluid in the chamber; c. at least one of the inner tube and outer tubeincluding the agglomeration or aggregation component; d. so that a mator sheet of the nanostructures can be formed on the agglomeration oraggregation component with a degree of alignment of the elongated axesof the nanostructures.
 4. The apparatus of claim 3 wherein only theouter tube rotates.
 5. The apparatus of claim 4 wherein the actuator isadapted to rotate the outer tube in the range of approximately 1 rpm to2000 rpm.
 6. The apparatus of claim 4 wherein only the inner tubeincludes the agglomeration or aggregation component.
 7. The apparatus ofclaim 3 wherein the agglomeration or aggregation component comprises aporous section positioned along the inner tube and a filtering mediumpositionable over at least an area of the porous section.
 8. Theapparatus of claim 7 wherein the filtering medium has characteristics topass the fluid of the dispersion but filter out at least substantiallythe nanostructures of the dispersion, and the mat or sheet is formed onthe filtering medium.
 9. The apparatus of claim 7 further comprising: a.a fill inlet to the chamber for supply of the dispersion; and b. anoutlet to capture the remainder of the filtered dispersion afterfiltering through the filter medium and the porous section.